The present invention describes an environment-friendly and economic process for preparing azo colorants.
In the context of the present invention, azo colorants are those azo dyes and azo pigments that are prepared by azo coupling reaction from a diazonium salt and a CH-acidic compound, referred to inter alia as coupling component hereinbelow (Ullmann""s Encyclopedia of Industrial Chemistry, Sixth Edition, xe2x80x9cAzo Dyesxe2x80x9d and xe2x80x9cAzo Pigmentsxe2x80x9d; and DIN 55943). Industrially, they are conventionally prepared by the batch process. Descriptions have also been given of continuous mixing processes in mixing nozzles and continuous flow reactors (in EP-A-0 244 686, for example).
A feature common to these processes is the need for precise monitoring and control of the process parameters: for example, temperature, time, mixing, and colorant concentrationxe2x80x94the suspension concentration in the case of azo pigments, for examplexe2x80x94are critical to the yield, quality consistency, and coloristic and fastness properties of the resulting azo colorants. In addition, in the case of batch processes, the scaleup of new products from laboratory to industrial scales is complex and may cause difficulties, since, for example, tank and stirrer geometries or heat transitions may greatly affect the azo pigment particle size and its distribution, and the coloristic properties.
It was an object of the present invention to find an environment-friendly, economic, technically reliable, and cost-effective process for preparing azo colorants by the azo coupling reaction, said process being universally suitable for the preparation both of azo pigments and of azo dyes; providing optimum mixing of the reactants; being combinable where appropriate with the measures known in connection with the preparation of azo colorants, such as the use of solvents or auxiliaries; permitting the desired process parameters to be maintained very constantly; and allowing easy scaleup.
The conduct of certain chemical reactions in microreactors is known (from DE-A-3 926 466, for example). Microreactors are constructed, for example, from stacks of grooved plates with microchannels and are described in DE 39 26 466 C2 and U.S. Pat. No. 5,534,328. U.S. Pat. No. 5,811,062 notes that microreactors are used preferentially for reactions which do not require or produce solids, since the microchannels easily become clogged.
It has now been found that the object of the invention may be achieved, surprisingly, through the use of a microjet reactor.
The present invention provides a process for preparing azo colorants which comprises spraying the reactants, i.e. the coupling component and the diazonium salt, in their solution or suspension form through nozzles to a point of conjoint collision in a reactor chamber enclosed by a housing in a microjet reactor, appropriately via one or more pumps, preferably high-pressure pumps, a gas or an evaporating liquid being passed into the reactor chamber through an opening in the housing for the purpose of maintaining a gas atmosphere in the reactor chamber, especially at the point of collision of the jets, and where appropriate of effecting cooling as well, and the resulting product solution or suspension and the gas or the evaporated liquid being removed from the reactor through a further opening in the housing by means of overpressure on the gas entry side or underpressure on the product and gas exit side.